Conventional silver halide photography uses silver halide particles as the light sensitive elements. These particles are only sensitive to ultraviolet and blue radiation. In order to make the silver halide sensitive to other wavelengths of the visible and infrared spectrum, sensitizing dyes are adsorbed to the surface of the silver halide particles. In order to enhance the sensitivity of the particles they are treated with so-called chemical sensitizers, such as sulfur compounds and gold compounds. The process of adding chemical sensitizers and dyes is known as chemical and spectral sensitization. It can be done in discreet steps or simultaneously (dye-in-the-finish) sensitization. There is a constant need for ever more sensitive emulsions. In the process of pushing silver halide particles to greater sensitivity through the use of sensitizing agents, it is difficult to achieve high sensitivity to light without introducing "fog" centers. Fog is the unwanted development of crystals which have not been exposed to light.
Fog is often controlled by the addition of emulsion stabilizers or antifoggants, but these materials must compete for the limited grain surface with all of the other addenda and dyes, limiting the total amount of addenda that can be used. It would be advantageous if a sensitizing dye also acted as an emulsion stabilizer and an antifoggant. This would allow higher dye coverage on the surface of the silver halide and less need for additional antifoggants and stabilizers.
U.S. Pat. Nos. 3,930,860 and 3,933,507 describes dyes that are antifogging through the formation of silver complexes, i.e. merocyanines with thiocarbonyl groups. One example of a 5,6-dimethoxybenzothiazole merocyanine is given in the latter reference. U.S. Pat. No. 4,493,520 generally alleges that certain spectral sensitizing dyes can also function as antifoggants or stabilizers. Reference is made to U.S. Pat. No. 3,930,860, discussed above, and U.S. Pat. No. 2,131,038, which discloses quaternary salts containing the nucleus: ##STR3## where Y=O, S or Se exert a stabilizing or anti-fogging effect. These compounds are only quaternary salts, not dyes. Therefore they compete for area on the surface of silver halide grains with the dye. With antifogging dyes less of these quaternary salts are needed.